Since coke used in a blast furnace is required to have a high strength, it is desirable to use a coal having a high caking property as a raw material for coke. However, all of mined coals do not have a high caking property; some of the mined coals may have a low caking property. Therefore, in common, plural types (brands) of coals having different properties are mixed together to form a coal blend, which is used as a raw material for coke. The caking property of a coal is an essential property for producing coke, which causes the coal to be melted and solidified in the carbonization of the coal. The caking property of a coal is determined on the basis of the properties of the coal when the coal is melted. Therefore, whether or not a brand of coal is suitable as a raw material for coke may be readily determined by effectively using a value on thermoplastic property (measured value or estimated value) of the coal as a measure.
In many cases, the higher the caking property of a coal, the higher the price of the coal; the lower the caking property of a coal, the lower the price of the coal. Thus, using a “non- or slightly caking coal” as a raw material for coke in a larger amount advantageously limits the costs of raw materials. However, it is not easy to evaluate the usability of a non- or slightly caking coal as a raw material for coke, because it is difficult (or, not possible) to measure a value on a thermoplastic property of a non- or slightly caking coal having a low (or, negligible) caking property by a method for testing the fluidity of coal by a Gieseler plastometer method defined in JIS M 8801, or a method for evaluating the caking property of a caking coal such as dilatometer method.
For the above context, a method for measuring (evaluating) the thermoplasticity of a non- or slightly caking coal has been developed. For example, it is described in Patent Literature 1 and Patent Literature 2 that the fluidity of a coal is measured by a Gieseler plastometer method under the conditions where the rate of temperature rise is set to 5° C./min or more, which is higher than the rate of temperature rise (3° C./min) specified in JIS M 8801, since the fluidity of a coal increases with an increase in the rate of temperature rise. In particular, it is described in Patent Literature 2 that the usability of a non- or slightly caking coal as a raw material for coke may be readily determined by the method proposed in Patent Literature 2, because there is a good correlation between the maximum fluidity (MF) of the non- or slightly caking coal which is measured with an increased rate of temperature rise and the CSR (coke strength after CO2 reaction) of a coke produced by the carbonization of a coal blend prepared by blending the non- or slightly caking coals.
In the method described in Patent Literature 3, the estimated fluidity of a noncaking coal is determined as a value on thermoplastic property of the noncaking coal. It is described in Patent Literature 3 that, in the above method, the fluidity A of a caking coal and the fluidity B of a coal blend that includes a noncaking coal and the caking coal are measured, and the estimated fluidity D that makes the fluidity B when added to the fluidity A is considered to be a value on thermoplastic property of the noncaking coal.